An arterial cannula is used to route oxygenated blood from a heart-lung machine to a patient's ascending aorta. The tip of the cannula must be sufficiently rigid so that it can be inserted through the incision without a dilator, and so that once it is inserted in the aorta it resists kinking. However, it is usually desirable for the cannula tip to be sufficiently flexible to minimize tissue damage if the tip scrapes the wall of the aorta (although some surgeons prefer rigid stainless steel cannula tips). It is also important to make the inside diameter of the cannula tip as large as possible relative to the outside diameter to minimize the pressure drop through the cannula and to minimize blood damage.
One type of arterial cannula currently in use is the "SARNS D.sup.4 " brand cannula sold by Sarns, Inc., of Ann Arbor, Mich., a subsidiary of the Minnesota Mining and Manufacturing Company of St. Paul, Minn. The tip of that cannula provides a desirable balance of rigidity and small pressure drop. The inlet conduit of that cannula tip is molded as a straight tube with a uniform wall thickness, which is then heated and bent. During the heat formation process the inside of the bend or the "chin" is subjected to high stresses, and sometimes wrinkles. This can cause weaknesses in the cannula tip, and a significant portion of the cannulas made this way must be rejected.
Other types of arterial cannulas include the cannula sold under the trade designation "ARGYLE THI AORTIC PERFUSION CANNULA" by the Sherwood Medical Company of St. Louis, Mo., and various arterial cannulas sold by C. R. Bard, Inc., of Murray Hill, N.J. It is believed that the Sherwood Medical Company produces its "ARGYLE THI" brand cannula by extrusion, and C. R. Bard produces their cannulas by repeatedly dipping the cannula tube into a molten elastomeric material until the desired wall thickness is achieved. Research Medical, Inc., of Salt Lake City, Utah, and DLP, Inc., of Grand Rapids, Mich., also produce arterial cannulas.